In order to prevent corrosion and/or other degradative forces from attacking pipelines, it is common practice to apply a protective cover to the surface of the pipe. While in its simplest form, this protective cover could be an anti-corrosion coating, for optimum longevity, especially for inground pipelines, the protective system will at least consist of an outer adhesive tape which is typically spirally wound around the pipe.
In a typical anti-corrosion system currently in use, a primer coating is first applied to the surface of the pipe and over this primer coating a pressure-sensitive tape having tough, abrasion-resistant backing is then wound. In some systems, a second tape is applied over the first tape.
Anti-corrosion protective tapes that are applied to inground pipeline structures are often subjected to rather severe long-term shearing forces derive from the surrounding soil. The magnitude of these shearing forces depends on several factors, including amongst others: (a) the type of soil, (b) the tectonic forces surrounding the implanted pipeline, (c) the size of the pipe, (d) the axial site emplacement and (e) the range of thermal expansion as well as its contents.
In order to understand how each of the above factors affected the overall shear stress imparted on an inground pipeline tape, we first shall consider the forces acting upon implanted pipelines.
Frictional forces acting between the pipeline anti-corrosion protective tape and the surrounding soil are the primary source of shear stress. Frictional forces are here defined as the product of the frictional coefficient between the pipeline coating and the soil and the normal forces acting upon the pipe. As the coefficient of friction depends upon both the nature of the pipeline coating as well as the surrounding soil, it will be found to vary in different applications. Olefin polymer pipeline protective coatings, such as polyethylene, or the like, inherently exhibit lower coefficients of friction, as the protective tape outer surfaces are smooth and substantially non-adherent.
Other factors having importance in these considerations are the weight of the pipe, including its contents. In addition, since the normal force will vary depending on the axial position around the pipe diameter, the frictional force and hence the shearing forces will also be found to vary around the diameter of the pipe.
The result of the long-term shear forces on a pipeline protective coating is referred to as "soil stress". Soil stress on an anti-corrosion protective coating generally results from the structural shear forces which cause the protective coating to creep along the pipeline peripheral surface.
Creep is, in essence, a long-term visco-elastic, or "cold-flow" phenomenon, common to all polymeric substances, The amount of creep, will depend upon physical properties of a coating. Since the physical properties (i.e. modulus) of a coating will be temperature dependent, temperature becomes a decisive element in determining the amount of creep. At low temperatures, the propensity of the protective coating to creep will be substantially reduced, while at elevated temperatures, the likelihood of creep will be significantly increased, other factors remaining the same.
The prior art has addressed these problems with various chemical approached directed to improving the cohesive nature of the adhesive, thereby increasing resistance to shear and creep.
Irrespective of the improvements provided by modifications of the chemical nature of the adhesive composition, it can be said that still further improvements in the protective tape system are desirable.
The present invention can in essence be described as a physical rather than chemical modification of the tape structure to achieve an improved pipewrap independent of the chemical components of adhesive itself.
The task of the present invention can accordingly be described as being directed to modifications in the tape configuration to obtain improvements in protecting pipewrap against degradative environmental forces for a given adhesive system.